Portable communication devices such as cell phones are becoming increasingly pervasive as their costs have come down and their functionalities have increased. Throughout this description the terms “cell phone”, “cellular phone”, “mobile phone”, “smart phone” and “wireless phone” are used interchangeably. Such devices provide mobile users with the convenience of having voice and data communication from most locations using a number of standards such as CDMA, TDMA, cellular second generation (2G), cellular third generation (3G), and cellular fourth generation (4G). Many phones also support other wireless standards such as WLAN 802.11*, Bluetooth®, Worldwide Interoperability for Microwave Access (WiMAX), 60 GHz, HD Radio™ and Ultra-WideBand (UWB). Phone users with data plans can use their cell phone to send and receive email, text messages, check their calendar, browse the Internet, upload and download content, and store contact information and other data. Cell phones also receive and send control data from and to base-stations, such as broadcast base-station beacon messages and unicast base-station data. Most phones are now equipped with still image and video cameras and take pictures and videos. High-end cell phones are also computing devices that download and run dedicated applications.
Cellular networks typically divide a coverage area into a series of cells. Each cell has a cell tower base-station that includes a transmitter, receiver, and antenna. The transmit power of the base-station determines the area of the cell; the larger the transmit power the larger the size of the cell. FIG. 1 illustrates a conventional cellular communication system according to prior art. As shown in the figure, cellular phone users 105 typically place the phone 110 next to their ear and their mouth near the microphone. When the user is making a voice call and is talking, the cell phone uses its antenna to transmit high frequency RF waves to one or more cell base-stations 115. The base-stations have base-station controllers and are connected to telephone networks that include operation centers, mobile services switching centers, and public switched telephone networks (as well as data networks such as the Internet). These components and networks route the voice call to the phone of the intended receiver. When receiving a voice call, the antenna of the cell phone receives radio frequency (RF) waves transmitted from the closest cell base-station. Cellular communication systems typically use a slightly different receive frequency, f2, versus transmit frequency f1, to avoid interference.
Several studies have been carried out to investigate the effect of cell phone radiation on the human body. A recent study and review in 2010 by the World Health Organization (WHO) categorized mobile phone use in the same carcinogenic hazard category as lead, engine exhaust and chloroform. An article by Danielle Dellorto, titled “WHO: Cell phone use can increase possible cancer risk,” published by CNN on May 31, 2011 discussed the results of the study. This article can be found on the Internet. The study concludes that in addition to causing brain cancer and tumors there could be other effects like cognitive memory function.
The RF radiation emitted from cell phones is similar to a low-powered microwave. The increased use of cellular phones for voice calls exposes humans to more potentially harmful RF radiation. Cell phones emit the most radiation when they are attempting to connect to cell base-stations because distances to the nearest base-station can be as high as 1-2 miles. A moving cell phone, or a cell phone in an area with a weak signal, has to transmit even more power and radiation. Several cell phone manufacturers recommend keeping the phones 15-25 mm away from the body when the phone is transmitting since the radiation decreases exponentially with distance. The pervasive use of cellular phones for voice calls, data transfer, and other functions also reduces their battery life.
Several attempts have been made to reduce the exposure of humans to RF radiation. One approach is to shield the body from the antenna radiation by placing a conductive strip between the transmitting antenna and the head of the user. Another approach is to move the RF radiation away from the body by changing the antenna location and radiation pattern. Another approach is to cancel the radiation pattern with an absorbent directional shield. Others have replaced wired or RF wireless headsets with an optical ear piece that connects the phone to the ear and shields the RF radiation, which is directed towards the user.
Wired and wireless headsets are another approach, where the cell phone is kept away from the human body. Here the user wears a headset that has an earpiece and a microphone, and the cell phone communicate with a headset using a wire or a short-range wireless protocol (e.g., Bluetooth®, 802.11*). There are however conflicting reports that indicate in some instances the headset acts as an antenna that focuses the radiation to the head and brain of the user. Also, unlike cell phones that are placed near the head only when they are in use, people who use headsets usually keep the headset on even when there are no voice calls and are therefore exposed to background signals such as the repeating beacon signals that are transmitted between a wireless headset and the cell phone. Furthermore, many people do not like using a headset and use the cell phone by placing it next to their ear and mouth.
Cellular repeaters are another approach. Repeaters are typically used to extend the coverage of cellular radio networks in difficult areas such as tunnels, and mountainous terrain. Indoor coverage of cellular networks in homes, shopping malls and small businesses are also improved with the use of smaller cellular base-stations called femtocells or microcells. Although these repeaters are bi-directional their primary design consideration is to boost the downlink signals from cellular base-stations to mobile devices. These repeaters are complex and expensive devices because they perform modulation/demodulation and other tasks. Although the cost of these devices is lower than installing cellular base-stations they are too expensive for widespread consumer use. They are also too bulky to be mobile and carried around by consumers.